Method of producing glass

ABSTRACT

An improved method is described for the production of glass by the decomposition of a mixture of glass forming compounds in the flame of a combustible gas to form an oxide mixture which is vitrified. The improvement includes generating vapors from a decomposable solid material that has a vapor pressure equal to 5 mm. Hg at a temperature above ambient temperature but not over 500*C. The vapors are generated by suspending the material in finely divided form in a heated chamber and passing a carrier gas through the material. The method is especially useful in the production of modified vitreous silica glasses.

United States Patent Schultz et a1.

[ Apr. 2, 1974 METHOD OF PRODUCING GLASS [73] Assignee: Corning GlassWorks, Corning,

[22] Filed: Dec. 15, 1971 [21] Appl. No.: 208,168

Nordberg 106/52 FROM GAS SUPPLY 2,823,982 2/1958 Saladin et a1. 423/3373,043,660 7/1962 Hughes et al. 423/337 3,486,913 12/1969 Zirngibl et al,423/337 X Primary Examiner-Robert L. Lindsay, J L Attorney, Agent, orfirm Clinton S Janes, Jr.

[57] ABSTRACT An improved method is described for the production ofglass by the decomposition of a mixture of glass forming compounds inthe flame of a combustible gas to form an oxide mixture which isvitrified. The improvement includes generating vapors from adecomposable solid material that has-a vapor pressure equal to 5 mm. Hgat a temperature above ambient temperature but not over 500C. The vaporsare generated by suspending the material in finely divided form in aheated chamber and passing a carrier gas through the material. Themethod is especially useful in the productionof modified vitreous silicaglasses.

7 Claims, 1 Drawing Figure PAIENTEDAPR 2 new: I 3.801. 294

FROM GAS 5 SUPPLY IO 60 I6 nv VENTOR$Z Peter C. Schultz Francis W.Voorhees -METHOD OF PRODUCING GLASS This invention is an improvement inthe method of making glass wherein a gaseous mixture of glass formingcompounds is decomposed in a flame of combustible gas to produce amixture of finely divided oxides which is vitrified. The improvement isparticularly concerned with a novel method of generating vapors of solidcompounds which have extremely low vapor pressures under ambientconditions.

U.S. Pat. No. 2,272,342, granted to J. F. Hyde on Feb. 10, 1942,describes a method of producing a transparent silica article byvaporizing a hydrolyzable compound of silicon into a flame ofcombustible gas to form finely divided silica particles which are thenvitrified. The vitrification may take place as the particles aredeposited, or may constitute a subsequent and separate heat treatment. Anumber of different hydrolyzable compounds containing silicon aredisclosed including silicon chloride. Vapors are generated by heatingthe liquid in a water bath, and a carrier gas may be passed through theliquid.

U.S. Pat. No. 2,326,059, granted to M. E. Nordberg on Aug. 3, 1943,describes a modification of the Hyde process wherein mixtures ofhydrolyzable compounds, in particular mixtures of titanium and siliconchlorides, are employed to produce SiO -TiO glasses. The halide vaporsare generated separately from heated flasks and mixed in a delivery tubeor generated from a liquid mixture in a single flask. The vapor mixtureis then delivered to a flame of combustible gas for decomposition of thehydrolyzed vapor mixture to a corresponding oxide mixture.

The procedure of the Nordberg patent has proven very useful in theproduction of SiO -TiO glasses having extremely low coefficients ofthermal expansion. However, this vapor delivery technique is limited inits application because of the limited availability of readilyvaporizable compounds. Thus, the compounds of many cations, e.g., thechlorides of zirconium, aluminum, tantalum and niobium, have extremelylow vapor pressures at ambient temperatures, and even at temperatures upto the boiling point of water (100C.).

U.S. Pat. No. 2,239,55 l granted Apr. 22, l94l to R. H. Dalton et al.,describes a method of producing a mixture of hydrolyzable compounds thatare decomposable to oxides wherein at least one of the compounds has alow vapor pressure under normal conditions. In this method, the desiredcompound is placed in a crucible in powder form and heated in a furnaceto a temperature at which vapors form. The vapors from the crucible maybe picked up by a second compound, such as silicon chloride, or by acarrier gas passed over the crucible.

In practice, we have encountered certain unsatisfactory results usingthe Dalton et al. technique. Apparently, only a limited amount of vaporis generated from the small exposed surface area of the vaporizablecompound and picked up by the gas passing over the crucible in thefurnace. The vapor yield can be increased by heating the furnace to atemperature at which the compound melts, or even higher, but thisnecessitates maintaining such elevated temperature throughout the systemto avoid condensation. More important, composition control becomesdifficult because the vapor pressure varies radically with smalltemperature changes at such elevated temperatures.

A primary purpose of the present invention is to provide a method ofproducing oxides by flame hydrolysis that eliminates these priorproblems. A more specific purpose is to provide a method of generatingrelatively large and controlled quantities of hydrolyzable vapors fromsolids that have a very low vapor pressure at ambient temperature. Afurther purpose is to provide an improved method of producing mixedoxide glasses by the flame hydrolysis technique. Another purpose is toprovide a wider range of modified vitreous silica glasses by the flamehydrolysis technique than has heretofore been practical to produce.

We have now discovered a method of generating copious quantities ofvapor from a solid that has a vapor pressure less than 5 mm. Hg atambient temperature (about 25C.), but at least 5 mm. Hg at 500C. Thisnovel method is based on the principle of passing a carrier gas through,rather than over, a vaporizable solid. We have found that this providesmuch greater surface area contact between the carrier gas and the solid,and thereby much greater vapor pick up from such contact.

On this basis, our invention is a method for producing a glass bydecomposing a mixture of glass forming compounds in a flame ofcombustible gas to form an oxide mixture and vitrifying the resultantoxide mixture comprising the steps of suspending a finely dividedmaterial within a chamber, said finely divided material including atleast one decomposable glassmaking compound having a vapor pressure of 5mm. Hg at a temperature about ambient temperature but not over 500C.,heating the chamber to an elevated temperature not over 500C., andpassing a carrier gas through the finely divided material to pick up andcarry vapors thereof to the flame of combustible gas. As in prior flamehydrolysis methods, the oxides formed may be deposited directly as asolid vitreous mass. Alternatively, they may be deposited in particulateform and subsequently consolidated to a solid mass by suitable heattreatment.

The finely divided material is preferably suspended in thin layers on aseries or plurality of perforated support members, or screens, spaced atdifferent levels in a vertical, closed container. Alternatively, a layeron a single screen may be adequate for some purposes. The carrier gaswill generally be passed upwardly through the material as a matter ofconvenience, but it is contemplated that the gas may be passeddownwardly as well if this proves desirable. Finally, the container maybe positioned horizontally in which case the divided material may fillthe container, or may be held in one or more thin layers by opposedscreens.

The particle size is not critical, and optimum size will increase withthe size of the apparatus used. In general, too fine a material willinterfere with gas passage whereas too coarse a material will notprovide sufficient surface area for vapor generation. In operating asmall system adapted to produce approximately one half pound of oxideper hour from the hydrolysis process, we find particles having sizes inthe range of 50 to 2,500 microns satisfactory, but prefer a particlesize range of to 500 microns. A larger system would use a somewhathigher range.

The present method may employ any material having a vapor pressure equalto 5 mm. Hg at a temperature above ambient temperature, but not overabout 500C. It has proven particularly useful in producing vapors fromcompounds which have a vapor pressure of 5 mm. Hg at a temperature inthe range 100C. to 500C.

Materials having a vapor pressure equal to 5 mm. Hg at a temperaturebetween ambient and 100C. may generate sufficient vapor without heating,or may be heated in a water bath in accordance with prior art methodsdescribed above. Materials having a vapor pressure less than 5 mm. at500C. generally do not develop sufficient vapor pressure below theirmelting temperature in order to permit use of the material by thepresent technique. Where such higher melting temperature materials areemployed, it is desirable to maintain the material in a molten state ina specially heated system and to pass a carrier gas through the moltenbath to carry off vapors generated therefrom.

Any material having a sufficiently high vapor pressure below its meltingpoint may be used. In addition to halides, these include such compoundsas organometallics. However, the metal halides are particularlysuitable, in particular the chlorides. By way of illustration, mentionmay be made of aluminum chloride (AlCl tantalum chloride (TaCl zirconiumchloride (ZrCl tungsten chloride (WCI beryllium chloride (BeCl tinchloride (SnCl and iron chloride (FeCl Normally, a gas not involved inthe hydrolysis process is used as a carrier gas. Nitrogen and oxygen areparticularly useful, but any gas which will not react with the compoundbeing vaporized can be used. In some instances, a separate vapor stream,e.g., a carrier gas carrying silicon halide vapor, may be used as thepick up gas stream that is passed through the finely divided material.However, condensation problems, as well as proportion control problems,usually make this impractical. It is also possible to employ a stream ofthe combustible gas used in the burner as the carrier gas. We findthough that this may lead to premature oxide deposition at the burnertip, and consequent clogging. Therefore, such practice is usuallyavoided.

In the preferred method for carrying out the invention, a verticalcontainer is provided with a series of screens, or otherwise suitablyperforated support members, that are vertically spaced, preferablyequidistant, within the chamber. While a single support member may beemployed, it is generally desirable to employ a plurality. A suitablyselected, vaporizable material is placed on each support or screen infinely divided form and the chamber is heated to a temperature at whichthis material has a vapor pressure greater than 5 mm., the temperaturenot exceeding 500C. A carrier gas is then introduced into the bottom ofthe chamber and allowed to pass through the finely divided solid thuspicking up vapors generated from the surface of such material. Thevapors picked up may then be mixed with vapors from one or more othersources and the mixture conveyed to a flame of combustible gas where thevapor mixture is hydrolyzed and decomposed to the desired oxide mixturein accordance with known flame hydrolysis technique.

The invention is further described, for illustrative purposes, withrespect to a specific embodiment thereof and with reference to theaccompanying drawing which illustrates a vapor generator and deliverysystem suitable for use in practice of the invention.

The drawing illustrates, schematically, an apparatus composed of threegeneral subassemblies, a vapor generator A, a delivery system B, and aburner C- The delivery system and burner are essentially conventional innature and hence are described only in general tenns.

Vapor generator A comprises a tube furnace 10 including an innerrefractory core 12, an electric heating element 14 wound on the outersurface of core 12, an outer shell 16 preferably composed of stainlesssteel, and an insulating material 18 packed between shell 16 and core12. Tube furnace 10 is closed at one end with bottom plate 20 and at theother end with top plate 22. Each of these may be composed of pressedasbestos board, or other suitable heat insulating material.

Vapor generator A further includes generating chamber 24 which rests onrefractory block 26. The latter may snugly fit within the bottom end ofcore 12 and rest against bottom plate 20 of tube furnace l0. Generatingchamber 24 includes a tubular shell 28, a bottom plate 30, and a topplate 32, each of which may be composed of stainless steel. Bottom plate30 is provided with a central opening 34 into which is fitted a tubingconnection 36 to connect inlet tube 38 with the chamber. lnlet tube 38has a coiled portion 40 having a dual purpose to be described later. Topplate 32 is also fitted with a pipe connection to connect chamber 24with delivery system B through outlet tube 42.

Vapor generating chamber 24 is provided with perforated support plates44 which may for example be composed of mesh stainless steel screen. Asexplained earlier, a single support plate may suffice for some purposes,but generally a suitable apparatus will include a plurality as hereshown. Desirably a porous metal plate 46, e.g., a 30 micron porousnickel plate, is provided near the bottom of shell 28. Plates 44 and 46fit snugly within shell 28 and are spaced apart, preferablyequidistantly, by steel ring bushings 48. This insures against gasleakage around the edges, and a tightly sealed chamber may be furtherinsured by providing end plates 30 and 32 with synthetic rubber 0- rings(not shown) capable of withstanding a temperature of 500C.

Vapor delivery system B includes a length of steel tubing 50 connectedto outlet delivery tube 42 with a valve 52. Tube 50 joins with tube 54at T-joint 56, tube 54 being connected through valve 58 with a secondgenerator not shown. T-joint 56 is connected to burner C by tube 60. Theburner may for example be a conventional gas oxygen burner adapted togenerate a hot combustible gas flame 62 in which the vapors hydrolyze.

In operation, vapor generator chamber 24 is so designed that the pipeconnections may be disassembled and the chamber removed from thefurnace. In turn, each of support plates 44 may be removed and coveredwith a layer of suitably selected finely divided material 64 to bevaporized. Chamber 24 is then reassembled with plates 44, and porousmetal plate 46 if present, spaced apart by spacers 48. The generatingchamber is then reinserted into the tubular furnace with the pipeconnections being remade.

Furnace 10 is then operated at a selected tempera ture in the range upto 500C, depending on the material selected. A carrier gas, such as drynitrogen or oxygen, is introduced through inlet tube 38. The coiledportion 40 of inlet tube 38 serves the dual purpose of providing aphysical support for generating chamber 24 and also providing preheatingof the carrier gas introduced through inlet tube 38. If porous metalplate 46 is present, it serves to evenly disperse the carrier gas flowlaterally across chamber 24. The carrier gas then passes upwardlythrough the heated particulate material 64 resting on the perforatedplates 44 and picks up quantities of vapor therefrom. The vapor-carriergas carried such vapors through delivery system B to T- joint 56 wherethe ZrCl mixture met a stream of silicon tetrachloride (SiCl )-oxygengenerated independently and introduced at a flow rate of 1,700 cc./min.

Vaporized during glass deposition; therefore percent content lower thanexpected.

By way of further illustrating practice of the invention with theparticular apparatus shown in the drawing, finely divided zirconiumchloride (ZrCl was loaded on screens 44 to a depth of approximately 1/8:inch. lnlet and outlet delivery tubes were attached and the vaporgenerating assembly heated to a temperature of 252C. within tube furnace10. This material has a vapor pressure of only atmosphere (0.00076 mm.Hg) at 100C, but has a vapor pressure of mm. Hg

at 252C.

It may be noted at this point that outlet tube 42 and delivery system Bmust be thermally insulated and maintained at a temperature at, orpreferably above, the operating temperature of the generating assembly.This is necessary to avoid condensation in the line. In the presentoperation, outlet tube 42 and delivery system B, as well as the tubingconnection to the burner, were maintained at a temperature of 290C.while burner C was maintained at 275C.

The burner flame was lit to provide moisture and heat for properhydrolysis of the incoming vapors when valve 52 was opened. Meanwhile,oxygen was being supplied through delivery tube 38 at a flow rate of2,900 cc./min., this gas being preheated in coil 40 before enteringchamber 24. As the carrier passed up- "wardly through the zirconiumtetrachloride powder, it

became saturated with vapors from this powder and mixture then passesout through outlet tube 42. V 7 5 by valve 58. The mixture thus producedwas passed The flow of vapor into delivery system B may be suitmtoburner C whre hydrolysls and decomppsition ably controlled by valve 52.Likewise, the flow of a sectook place. to provide resultant Soot orpamculate 0nd vapor, such as silicon chloride (SiCl from a powdeimlxnire contammg 40 Zroz and Source not Shown may be controlled by valve58. The wt.% S10 This powder was collected on a mandrel and two vaporstreams thus controlled meet and mix into a the form thusfproduced Yconsoildated mm a transsingle stream at T-joint 56, thereafter passingthrough m 0 good quanta by heatmg the preform for tube 60 to burner C.In known manner, the vapors are g i z i hydrolyzed in the burnerassembly to form the corredata 2 j z g rfix z g i mg sponding oxides inflame 62 of combustible gas promm w vapors 15 were generated and mixedwith SIC] vapors to produce vided by the burner. r a a 1 I 7 a mixedoxide glass in accordance with the present in- If burner C is suitablyadjusted to a sufficiently high vention. The table lists the compoundemployed; the temperature, the oxide mixture 66 delivered fromrespective temperatures in C. at which chamber 24, flame 62 will be invitreous form and may be collected line 42 and burner C were maintained;the vapor presin any suitable manner to form a glass article. If flame20 sure in mm. Hg of the compound at the chamber tem- 62 is adjusted toa somewhat lower temperature, the perature; the flow rate of carrier gasin vapor generator oxide mixture emanating from flame 62 may be in par-A in cc./min.; the corresponding flow rate of SiCl. -ticulate, powderform and may be deposited on a suitthrough valve 58; and, finally, thecomposition, in perable mandrel or other support to form a dense solidcent by weight on an oxide basis, of the glass produced body. This inturn may subsequently be vitrified in 25 (only the additive oxide beinggiven, the remainder known manner by suitable heat treatment. being SiOTABLE Temperature C. Flow Rates (ec.lmin.)

Oxide Compound Chamber Line Burner VP(mm.Hg) Compound SiCl Wt.%

AlCl 139 190 190 40 2650 1340 5.8 TaCl 300 240 15 1300 3450 3.8 mm, 285280 25 2350 3400 3.3 MoCl, 140 330 255 15 2350 2180 .0l0* Zrcl, 252 290275 30 2900 l700 4.0 BeCl, 300 350 315 5 1850 i640 4.8 wct, 218 290 24040 2350 I930 06* The foregoing table illustrates the use of severaldifferent suitable chlorides for the purposes of the invention. Ifdesired, one working in this art can readily substitute othervaporizable metal compounds having similar characteristics.

For example, zirconium iodide (Zrl has a vapor pressure of 30 mm. of Hgat 339C. and zirconium bromide (ZrBr has a vapor pressure of 30 mm. ofHg at 276C. lf one suitably adjusts the chamber, line, and burnertemperatures, and maintains flow rates as indicated in the above table,either of these compounds may be substituted for ZrCl. to produce theglass composition shown in the table.

Likewise, beryllium bromide (BeBr has a vapor pressure of 5 mm. of Hg at325C. This compound could then be substituted for Becl to produce theSi- O -BeO glass composition shown in the table by increasing thechamber, line, and burner temperatures 25C. each and maintaining theindicated flow rates.

Attempts to employ this method with other compounds, in which a vaporpressure of 5 mm. Hg occurs above 500C., have proven generallyunsuccessful. For example, MgCl NiCl and MnCl have been tried withoutsuccess. These, respectively, have vapor pressures of 5 mm. Hg at about877C., 731C. and 736C.

It will be appreciated that, while a specific apparatus and a specificmaterial mixture have been described,

this has been solely for purposes of illustration and the scope of theinvention is limited only by the claims that follow. In particular, theinvention may be practiced by generating two or more separate sources ofmaterial in generators such as illustrated by generator 24 andthereafter mixing the vapor streams generated. In some cases, it is alsopossible to properly proportion a mixture of solid materials, on thebasis of their respective vapor pressures, so that a suitable vapormixture may be picked up by a single stream of carrier gas. Also, asindicated earlier, a wide variety of vaporizable and hydrolyzablematerials may be employed in practice of the present invention. Finally,it is well within the skill of the art to devise various modificationsof the specific elements shown in the apparatus above while stillemploying the principles of the invention.

We claim:

1. In a method for producing a glass body wherein a mixture of compoundswhich are capable of being bydrolyzed and decomposed into glass formingoxides is introduced into a flame of combustible gas to formfinely-divided particles which are vitrifiedandwherein at least one ofsaid compounds has a vapor pressure of mm. Hg at a temperature aboveambient temperature but not over about 500C, the improvement whichcomprises generating vapors from said compound comprising the steps of:

a. suspending said compound in finely-divided particulate form within achamber;

b. heating said compound within said chamber to a temperature aboveambient temperature but not over 500C. to generate vapors thereof; andthen c. passing a carrier gas through said compound in finely-dividedform to pick-up and carry vapors thereof to a flame of combustible gas.I V

2. The method of claim 1 wherein the glass produced is a modifiedvitreous silica glass.

3. The method of claim 1 wherein the mixture of glass forming compoundsdecomposed in a flame is a mixture of halides.

4. The method of claim 3 wherein the mixture of halides includes siliconhalide and at least one other halide.

5. The method of claim 1 wherein the oxide mixture formed in the flameis deposited on a support in particulate form and the body thus formedis subsequently vitrified by heat treatment.

6. The method of claim 1 wherein the oxide mixture is vitrified in theflame and is deposited in vitreous form.

7. A method in accordance with claim 1 wherein silicon halide vapors aregenerated separately by passing a carrier gas through silicon halide andthe stream of gas and vapors thus formed is mixed with the stream of gasand vapors generated in the heated chamber and the mixture is passedthrough the flame of combustible gas.

2. The method of claim 1 wherein the glass produced is a modified vitreous silica glass.
 3. The method of claim 1 wherein the mixture of glass forming compounds decomposed in a flame is a mixture of halides.
 4. The method of claim 3 wherein the mixture of halides includes silicon halide and at least one other halide.
 5. The method of claim 1 wherein the oxide mixture formed in the flame is deposited on a support in particulate form and the body thus formed is subsequently vitrified by heat treatment.
 6. The method of claim 1 wherein the oxide mixture is vitrified in the flame and is deposited in vitreous form.
 7. A method in accordance with claim 1 wherein silicon halide vapors are generated separately by passing a carrier gas through silicon halide and the stream of gas and vapors thus formed is mixed with the stream of gas and vapors generated in the heated chamber and the mixture is passed through the flame of combustible gas. 